The idea of function-retaining artificial replacements for intervertebral discs is younger than that for replacements of artificial joints of extremities, but nonetheless about 50 years old [Büttner-Janz, Hochschuler, McAfee (Eds.): The Artificial Disc. Springer Verlag, Berlin, Heidelberg, N.Y. 2003]. It results from biomechanical considerations, unsatisfactory results of fusion surgeries, disorders adjacent to fusion segments and the development of new materials with greater longevity.
The publications and other materials, including patents, used herein to illustrate the invention and, in particular, to provide additional details respecting the practice are incorporated herein by reference.
By means of function-retaining disc implants, it is possible to avoid fusion surgery, i.e. to maintain, or to restore the mobility within the intervertebral disc space. In an in-vitro experiment it is also possible to achieve a normalization of the biomechanical properties of the motion segment to a large extent through the implantation of an artificial intervertebral disc after a nucleotomy.
Implants for the replacement of the whole intervertebral disc differ from those for the replacement of the nucleus pulposus. Accordingly, implants for the total replacement of the intervertebral disc are voluminous; they are implanted via a ventral approach. An implantation of a prosthesis for total replacement of the intervertebral disc immediately after a standard nucleotomy can therefore not be carried out.
The indication for a function-retaining intervertebral disc replacement as an alternative to the surgical fusion includes, besides the painful discopathy, also pre-operated patients with a so-called post discectomy syndrome, patients with a recurrent herniated intervertebral disc within the same segment and patients having a pathology within the neighboring intervertebral disc as a consequence fusion surgery.
Presently, a total of more than 10 different prostheses are clinically used for the total replacement of intervertebral discs. For the lumbar spine the, CHARITÉ Artificial Disc, the PRODISC, the MAVERICK, the FLEXICORE and the MOBIDISC (Overview in Clinical Reports, PJB Publications Ltd., June 2004) are particularly well known, and for the cervical spine the BRYAN prosthesis, the PRESTIGE LP prosthesis, the PRODISC-C and the PCM prosthesis, which will be described below.
The PRODISC prosthesis for the lumbar spine is being implanted since 1999, following its further development to the PRODISC II. The PRODISC prosthesis has been in use as a lumbar spine implant since 1999, until it was replaced by the further developed PRODISC II. Although with respect to its components (a three-part intervertebral disc prosthesis), it is functionally a two-part prosthesis with its sliding partners made of metal and polyethylene. Implantations of the PRODISC are carried out in the lumbar spine and with an adapted model of the prosthesis, the PRODISC-C, also in the cervical spine. Different sizes, heights (achieved by the polyethylene core) and angles of lordosis (achieved by the metal endplates) are available. Bending forward and backward as well as to the right and to the left is possible to the same extent of motion; the axial rotation is not limited in the construction.
The same applies to both two-part prostheses for the cervical spine, the PCM prosthesis with its sliding partners metal and polyethylene and the PRESTIGE LP prosthesis with its sliding partners metal. As special feature of the construction of the PRESTIGE LP prosthesis it has the possibility for an anterior-posterior translation, due to the horizontal ventrally prolonged concavity, which, in a frontal section, has the same radius as the convexity.
The MAVERICK and the FLEXICORE for the lumbar spine are functionally a two-part prostheses with spherical convex-concave sliding partners, both with sliding partners made of metal. In contrast, the MOBIDISC is functionally a three-part prosthesis with sliding partners of metal-polyethylene and two articulation surfaces. One area is a segment of a sphere, as it is in the three afore mentioned prostheses, with a convex and a concave surface of the articulating partners each of the same radius, the other area of the MOBIDISC being plane. Although a limitation of the axial rotation is planned within the plane section, it is not limited within the convex-concave area of articulation. In contrast the FLEXICORE has a small stopping area within the spherical sliding surfaces limiting the rotation movement.
The BRYAN prosthesis is clinically used as a compact prosthesis for total replacement of intervertebral discs of the cervical spine. It is attached to the vertebral bodies by convex titanium plates with a porous surface and achieves its biomechanical properties by virtue of a polyurethane nucleus.
The longest experience exists with the CHARITÉ prosthesis, which is the subject matter of DE 35 29 761 C2 and U.S. Pat. No. 5,401,269. This prosthesis was developed in 1982 by Dr. Schellnack and Dr. Büttner-Janz at the Charité in Berlin and was later on named SB CHARITÉ prosthesis. In 1984 the first surgery took place. The intervertebral disc prosthesis was further developed into model III and has been implanted over 10,000 times worldwide (DE 35 29 761 C2, U.S. Pat. No. 5,401,269) since 1987 and is still being used. The prosthesis is functionally a three-parted being with the sliding partners being metal and polyethylene with two identical spherical sliding surfaces. It has a transversally mobile polyethylene core and the accordingly adapted concave cups within two metal endplates. For the adaptation to the intervertebral space, the CHARITÉ prosthesis provides different sizes of metal plates and different heights of size adapted sliding cores as well as angled prosthetic endplates, which when implanted vice versa in sagittal direction can also be used as replacement for the vertebral body. The primary fixation of the CHARITÉ prosthesis is achieved by six teeth, which are located in groups of three slightly towards the middle next to the frontal and rear edge of each prosthetic plate.
The other prosthesis have other primary fixations on their surfaces directed towards the intervertebral bodies, e.g. a sagitally running keel, a structured surface, a convex shape with for instance crosswise running grooves and combinations thereof, also with differently located teeth. Furthermore screw fixations can be used, either from ventral or from within the intervertebral space into the intervertebral body.
To assure a long-term fixation of the prosthetic endplates to the intervertebral bodies and to thus generate a firm connection with the bone, a surface was created in similitude to cement-free hip and knee prostheses, which combines chrome-cobalt, titanium and calcium phosphate in such a way that it is possible for bone to grow directly onto the endplates. This direct connection between prosthesis and bone, without the development of connective tissue, makes a long-term fixation of the artificial intervertebral disc possible and reduces the danger of loosening or displacements of the prosthesis and material breakage.
One primary objective of function retaining intervertebral disc replacements is to closely adapt the motions of the prosthesis to the ones of a healthy intervertebral disc. Directly connected to this is the motion and stress for the facet joints, which following inappropriate biomechanical stress have their own potential for disorders. There can be abrasion of the facet joints (arthritis, spondylarthritis), a formation of osteophytes. As result of these osteophytes and also by a pathologic course of motion of the intervertebral disc alone, the irritation of neural structures is possible.
The healthy intervertebral disc is, in its interactions with other elements of the motion segment, composed in such a way that it allows for limited motion. For example, within the intervertebral disc, motions to the front and back are combined with rotary motions, and side motions are also combined with other motions. The motion amplitudes of a healthy intervertebral disc are very different, with respect to the extension (bending back) and flexion (bending forward) as well as to the lateral bending (right and left) and rotary motion. Although of common basic characteristics, there are differences between the motion amplitudes of the lumbar and cervical spine.
During motion of the intervertebral disc the centre of rotation changes, i.e. the motion of the intervertebral disc does not take place around a fixed center. Due to a simultaneous translation movement of the adjacent vertebrae, the center changes its position constantly (inconstant center of rotation). The prosthesis according to DE 35 29 761 C2 shows a construction which differs relative to other available types of prostheses which are build like a ball and socket joint and as a result move around a defined localized centre of rotation. By virtue of the three-part assembly of the prosthesis according to DE 35 29 761 C2, with two metallic endplates and the interpositioned freely mobile polyethylene sliding core, the course of motion of a healthy intervertebral disc of the human spine is mimicked as far as possible, however without the exact motion amplitudes in the specific motion directions.
A further important feature of the healthy lumbar intervertebral disc is its trapezium shape, which is primarily responsible for the lordosis of the lumbar and cervical spine. The vertebral bodies themselves contribute only to a minor extent to the lordosis. During prosthetic replacement of intervertebral discs the lordosis should be maintained or reconstructed. The CHARITÉ disc prosthesis provides four differently angled endplates, which moreover can be combined with each other. However, this surgery requires more surgical effort and has the risk of damaging the vertebral endplates which is associated with a danger of subsidence of the prosthesis into the vertebral bodies. Additionally, if the adjustment of the lordosis is poor and an optimal load of the center of the polyethylene core was not achieved, the prosthesis has to be removed completely.
To avoid sliding or a slip-out of the middle sliding partner from the endplates, DE 35 29 761 C2 discloses a sliding core with a two-sided partly spherical surface (lenticular), with a plane leading edge and at the exterior with a ring bulge, which will lock between the form-adapted endplates during extreme motion. DE 102 42 329 A1 discloses a similar intervertebral disc prosthesis which has a groove around the contact surfaces, in which an elastic ring is embedded that is in contact with the opposite contact area for better guidance.
EP 0 560 141 B1 describes a three-part intervertebral disc prosthesis, which also comprises two endplates and an interpositioned prosthetic core. The intervertebral disc prosthesis, described in this document, provides resistance during rotation of its endplates in opposing directions around a vertical rotary axis without a contact between the prosthetic endplates. This is achieved by a soft limitation of the endplates during rotation onto the prosthesis core caused by the weight, which acts on the plates as a result of the biomechanical load transfer within the spine, because the corresponding radii of curvature differ in a median-sagittal and frontal transection.
The above mentioned models are permanently anchored in the intervertebral spaces as implants. Especially due to a load transfer over too small surface areas, a migration of the endplates into the vertebral bodies and thus a dislocation of the complete implant is possible in middle to long-term use, resulting in artificial stress for the vertebral bodies and the adjacent nerves and in the end for the total motion segment, and leading to new complaints of the patients. The longevity of the polyethylene also needs to be discussed because destruction of the sliding cores has been observed, which necessitated revision surgery, so far in the form of a fusion of the motion segment
The risk of postoperatively persisting complaints is higher if the facet joints of the surgical segment already show signs of arthritis at prosthetic implantation. It also has to be taken into account that a too large segmental range of motion, resulting from the design of the prosthesis, may potentially lead to new complaints for the patient. This is most likely caused by an overloading or malapropos stress on the facet joints, which may lead to painful arthritis. The same applies to prosthesis that have been implanted frontally inclined or that have postoperatively developed a malapropos positioning. Furthermore, fusion surgery leads to an increased strain on the neighboring segments with the danger of a later indication for surgery at this level. An intervertebral disc prosthesis with a segmental partial function may thus present a solution to this problem.
EP 1 039 855 B1 discloses a partially cylindrical implant for the intervertebral space. This implant has an elastic core, which is located between two end plates that are assembled to an upper and a lower vertebral body. Motion within the intervertebral space is only possible as far as the elastic core can be compressed.
U.S. Pat. No. 5,539,409 also discloses a partially cylindrical implant for the intervertebral space. Such as implant has a rough surface and, as per the invention, is to be filled with substances that will encourage the fusion of the implant with the bone of the neighboring vertebra. A motion of the affected segment of the spine after implantation is therefore not possible.
Furthermore, intervertebral disc prostheses, from the state of the art known, have one or more cylindrical compressible middle parts. An example can be found in CA 2 376 097 A1, which discloses a prosthesis comprising a cylindrical upper and lower hull, in between which a cylindrical middle part made of an elastic material is positioned.
In the intervertebral disc prostheses with a cylindrical core, known from the state of the art, this is mostly made of an elastic material or is firmly assembled to the neighboring vertebral body. U.S. Pat. No. 5,258,031 discloses a lateral section, partially cylindrical, articulation area of a two-part intervertebral disc prosthesis, which permits a bending to the sides via the lateral edges of the cylindrical marginal convexity, so that the load bearing on the endplates is partially only on the edges and so that an increased wear of these regions of the articulation areas is to be expected. Such a prosthesis can only be implanted by ventral surgery because of the size of the keels and/or the fixation of the prosthesis is by means of screwing.
There is a need for an intervertebral disc prosthesis for the total replacement of the intervertebral disc, which will enable a dorsoventral and a rotational motion of a spinal segment, but does not allow for sideways bending. It will be possible to implant the prosthesis by surgery from ventrolateral and lateral as well.
This need is addressed by the present invention. The invention comprises an intervertebral disc prosthesis, namely a functionally two-part prosthesis.